1. Field of the Invention
The present invention relates to a polyalkylene oxide based polymer composition for solid polymer electrolytes, and more particularly, to a polyalkylene oxide based polymer composition for solid polymer electrolytes, which has superior mechanical properties as well as ionic conductivity by comprising a cross-linking agent with at least two functional groups of phenyl alkyleneglycol acrylate substituted in core molecules; a softener of polyalkyleneglycol alkylether alkyl(metha)acrylate; a plasticizer of polyalkyleneglycol dialkylether; a curable initiator and lithium salt and thus can be used as an electrolyte for a high capacity lithium-polymer secondary battery for load leveling or electric vehicles as well as an electrolyte for a small capacity lithium-polymer secondary battery for portable information terminals such as a cellular phone and a notebook computer, and electronic products such as a camcorder.
2. Description of the Prior Art
Electrochemical devices using solid electrolytes have advantages over the conventional ones using liquid electrolytes because they are free of a liquid leakage problem used to be present in the conventional ones, can provide batteries with high recharge/discharge efficiency, and can be prepared in the form of a film and also in a small size. In particular, the lithium-polymer batteries using polyalkylene oxide (PAO) based solid polymer electrolytes can be used in manufacturing batteries with improved energy density and the importance of these batteries have been greatly emphasized.
PAO based solid polymer electrolytes was first developed by P. V. Wight in 1975 (British polymer Journal 7, p.319), named as xe2x80x9cion-conducting polymerxe2x80x9d by M. Armand in 1978, and its application into electrochemical apparatus is on the steady increase. Conventional solid polymer electrolytes form a complex comprising a polymer of electron-donor atoms, such as oxygen (O), nitrogen (N) and phosphorous (P), and a lithium salt. The well-known example of the above complex is the one prepared by polyethylene oxide (PEO) and a lithium salt. This complex shows such a low ion conductivity as 10xe2x88x928 S/cm and thus it is not suitable for room temperature use but can be used as a power source of electrochemical apparatus operated at high temperature.
PAO based solid polymer electrolytes have high crystallinity at room temperature thus resulting in low ion conductivity because the chain segmental motion is restricted. To increase the chain segmental motion of solid polymer electrolytes, the crystalline domain within the polymer structure should be minimized. A series of studies as such were conducted by introducing a relatively short PAO as a side chain thus forming a comb-like polymer or by introducing a cross-linkable functional group at the end of PAO thus forming a network structure. Such studies revealed that branched solid polymer electrolytes such as polybismethoxyethoxyethoxyphosphazine [J. Am. Chem. Soc., 106 (1984) 6845] or polybisethoxyethoxyethoxyvinylether [Electrochim. Acta, 34 (1989) 635] are also possible. U.S. Pat. No. 4,830,939 [J. Electrochem. Soc., 145, 1521 (1998)] disclosed a method of manufacturing cross-linked polymer electrolytes by UV or e-beam radiation curing of a composition wherein acrylate of polyalkylene glycol, ion-conducting liquid and electrolytic salt are mixed together. U.S. Pat. No. 5,240,791 by YUASA Co., Ltd. in Japan disclosed solid polymer electrolytes cross-linked between PEO monoacryloyl monomers and PEO diacryloyl monomers. These cross-linked solid polymer electrolytes are mostly cured by high energy beam irradiation such as e-beam or UV light. These electrolytes can be prepared as self-supportive films after being cured, however, they tend to be easily fractured due to the deficiency in stretching or bending properties thus making it difficult to be used on lithium-polymer batteries unless they are directly coated on the batteries. Unexamined Published Korean Patent Application No. 2001-4121 by the present inventors, as a way to solve the above problems, discloses a method to improve the mechanical property of the polymer electrolytes by using cross-linkers having rigid 6-membered cyclic molecules with three poly(ethyleneglycol) acrylates.
As described above, the conventional cross-linked solid polymer electrolytes can be prepared as self-supportive films after being cured, but they can be easily fractured due to the lack of stretching or bending mechanical properties thus not enabling to be applied into lithium-polymer batteries unless they are directly coated. In an effort to further solve this problem, the inventors of the present invention figured out that a new type of a cross-linking agent is essential in developing a solid polymer electrolyte with much improved physical property as well as ion conductivity which differs greatly from a vinyl cross-linking agent or an acrylate having a bifunctional group or more than two functional groups wherein the functional group is linear or branched carbon backbone. The conventional cross-linking agents were unable to impart enough mechanical strength because they are usually in the form of a linear or a branched carbon backbone, and the inventors of the present invention improved the mechanical strength by modifying the central structure of a given cross-linking agent and completed this invention. Therefore, the object of this invention is to employ a novel cross-linking agent useful in manufacturing solid polymer electrolytes of a three-dimensional network structure wherein the cross-linking agent has at least two phenyl alkyleneglycol acrylates in the center of the linking molecule. Another object of this invention is to strengthen the ion conductivity as well as the mechanical strength thus providing solid polymer electrolytes suitable to be used in lithium-polymer secondary batteries.